The present invention relates to fatigue monitoring and, more particularly, to a device for providing an indication of the progression of fatigue damage within a structure and a method for making such a device.
Potential structural failure due to fatigue constitutes one of the most troublesome areas of structural engineering primarily because fatigue failure occurs suddenly, usually in critical areas of a structure. Although many aspects of fatigue are still unknown, it is generally understood that the fatigue process starts with the microscopic imperfections or defects which are present in all materials. Under certain circumstances, the microscopic imperfections rapidly grow and coalesce to form a macroscopic defect in the form of a crack. The growth and propagation of macroscopic cracks is the immediate cause of a fatigue failure. However, the appearance of such macroscopic cracks occurs relatively late in the fatigue process and, therefore, cannot be used as an acceptable warning device of impending fatigue failure.
The primary factor which causes the inherent microscopic material defects to grow and coalesce is the presence of an intense stress or strain field (hereinafter collectively referred to as a "stress field"). Such intense stress fields or stress concentrations generally occur in the vicinity of sudden discontinuities or "stress raisers" such as holes, notches or other similar geometric discontinuities within a structural configuration. Thus, fatigue failure generally originates at or near such stress raisers and is believed to begin whenever a certain critical stress or critical strain is exceeded. Fatigue is therefore a primary design consideration in many applications which involve repeated and often varying loadings such as aircraft, machine elements, pressure vessels, bridges, etc.
In the past, structural designers have attempted to circumvent the problem of fatigue failure by designing structures in a manner which maintains the stresses present in the critical areas of a structure at a level well below the known endurance limits of the material employed. Hence, minimum radius holes, fillets etc. are introduced into structural designs and only "mild" stress raisers are employed in the structure in order to provide relatively smooth structures and to thereby decrease the likelihood of fatigue failure. While this type of design approach results in structures which are generally safe and relatively free of fatigue failure, it also results in unacceptable penalties in structural efficiency which, in turn, result in excessive structural costs.
When designing a structure to compensate for fatigue and in trying to assess the potential fatigue life of the structure, a designer generally relies on a combination of experimental data and previously developed empirical design rules. Experimental fatigue data have been accumulated on most structural materials in the form of S-n curves which provide an indication of the number of loading cycles which will induce fatigue failure as a function of the stress level applied to the material. The experimental data, however, exhibit a significant amount of scatter and are only strictly applicable to structures where the cyclic stress applied to the structure is of a constant amplitude. Most actual structures which are subjected to repeated loadings generally experience varying levels of stress for different numbers of cycles. Thus, the fatigue life of a particular structure greatly depends upon its specific individual stress history.
In applying the experimental S-n curve data to actual loading situations, empirical rules and procedures have been suggested. These empirical rules, referred to as cumulative damage rules, (the most common of which is Minor's Rule), have also been found to be inadequate. Even in the simplest case of two different stress levels applied to a structure, it has been demonstrated experimentally that structural fatigue life is dependent on the order of application of the stress levels. None of the known cumulative damage rules take into account the potential differences in the order of application to the structure of differing stress levels. Moreover, cumulative fatigue damage cannot be determined by non-destructive testing so that, short of conducting a detailed microscopic examination of the structure of the material, there is no known way to accurately determine the cumulative damage of a structural member at a given time.
In summary, in addition to being inefficient, the practice of designing structures to take into account fatigue is highly speculative since the actual loading history of the structure is not known and cannot be accurately predicted. Therefore, there is a need for a device which would monitor fatigue damage and provide a reliable estimate of the remaining fatigue life of a particular structure in order to provide a warning of impending fatigue failure in sufficient time to permit measures to be taken, such as the repair or replacement of such structures, to minimize the possibility of catastrophic consequences.
The present invention comprises a relatively simple fatigue monitoring device which can provide a repeatable, reliable estimate of the remaining fatigue life for any desired structural member. The device is attached to the structural member whose fatigue life is to be monitored so that the device is subjected to the exact same strain history and environment experienced by the actual structural member. The device is small and inexpensive to manufacture and can be specifically tailored to particular structural applications and structural materials as required.